Optical guide including nanoparticles and manufacturing method for a preform intended to be shaped into such an optical guide

ABSTRACT

The invention relates to an optical fiber comprising a gain medium which is equipped with: a core ( 22 ) which is formed from a transparent material and nanoparticles ( 24 ) comprising a doping element and at least one element for enhancing the use of said doping element; and an outer cladding ( 26 ) which surrounds the core. The invention is characterised in that the doping element is erbium (Er) and in that the enhancing element is selected from among antimony (Sb), bismuth (Bi) and a combination of antimony (Sb) and bismuth (Bi). 
     According to the invention, one such fiber is characterised in that the size of the nanoparticles is variable and is between 1 and 500 nanometers inclusive, and preferably greater than 20 nm.

FIELD OF THE INVENTION

The present invention pertains to an optical guide, and in particular anoptical fibre, that amplifies telecommunications signals, and amanufacturing method for a preform intended to be shaped into such anoptical guide.

PRIOR ART

It is known in the art how to use an optical fibre comprising a gainmedium to regenerate an optical signal received by that fibre and toretransmit the regenerated optical signal with increased intensity. Todo so, such an amplifying fibre comprises:

-   -   a core made up of a transparent material incorporating at least        one doping element such as rare earth ions like erbium (Er)        which amplify the optical signal, and    -   a cladding surrounding the core, intended to keep the majority        of the optical signal within the core.

Conventionally, incorporating doping elements and enhancement elementsis done by impregnating porous glass with solutions that include thesevarious doping agents in the form of dissolved salts. This method hasthe disadvantage of not allowing these elements to be incorporated in asatisfactory manner into the preform prior to the manufacture of a fibrewhen said manufacturing uses a method called MCVD, for Modified ChemicalVapour Deposition.

Indeed, the MCVD introduces high temperatures, which are incompatiblewith the high volatility of numerous elements and/or with the lowstability of compounds created using these elements.

Another manufacturing method, termed “Multicomponent Oxide Glass” orMOG, is also used to incorporate new elements into a fibre.

However, the MOG method uses traditional glassmaking, mixing componentsin a crucible and exposing them to a high-temperature heat treatmentthat particularly has the disadvantage of requiring complicated andcostly fibre production techniques.

Furthermore, the optical fibres produced using the MOG method have anoptical signal attenuation rate higher than the attenuation rate for afibre produced using the MCVD method, owing to the impurities introducedby crucible synthesis, and welding problems in relation to transmissionfibres which are manufactured with the MCVD method.

An amplifying fibre is described, to give one example in patentapplication US2003/0,175,003, which discloses the use of nanoparticlessmaller than 20 nm, which contain chemical elements in the vicinity ofthe doping element to improve signal amplification; said elements arehereafter termed enhancement elements.

This document also describes the organometallic synthesis of thesenanoparticles and their insertion into the fibre core using an MCVDmethod.

SUMMARY OF THE INVENTION

The present invention includes the observation that there exists a needfor a method for manufacturing amplifying fibres that makes it possibleto insert enhancement elements in the vicinity of the doping element inorder to maintain physical properties closes to those of a standardsilica fibre, and thereby to make it easier to weld the standard fibreto the amplifying fibre manufactured in this manner.

The subject of this invention is an optical fibre comprising a gainmedium equipped with a core (22) made up of a transparent material andof nanoparticles (24) which include a doping element and at least oneelement that enhances the use of said doping agent, and further of anouter cladding (26) surrounding the core, characterised in that thedoping element is erbium (Er) et and that the enhancement element ischosen from among antimony (Sb), bismuth (Bi) and a combination ofantimony (Sb) and bismuth (Bi).

An optical fibre created via the present invention uses new types ofnanoparticles comprised of antimony and/or bismuth. It thereby benefitsfrom the enhancing properties of bismuth and/or antimony when theseelements are in the vicinity of erbium.

Furthermore, the relatively large size of these nanoparticles, which isbetween 1 and 500 nm inclusive and is preferably greater than 20 nm,makes it possible to incorporate them into the fibre and keep them inplace, even when high temperatures are used to manufacture the guide,such as in an MCVD method.

In one embodiment, the doping element and/or the enhancement element ispresent in the form of an oxide. Thus, by creating the nanoparticle fromone doping element and/or one oxidised enhancement element, the risksthat the nanoparticle will be altered by oxidation of said elements arelimited.

In one embodiment, the guide includes aluminium in the core, near thenanoparticles, with this inclusion enhancing the properties of thenanoparticles.

The invention also relates to a method for manufacturing a preformintended to generate an optical fibre that includes a core, made up of atransparent matrix and of nanoparticles that comprise a doping elementand at least one element that enhances the use of said doping element,and an outer cladding surrounding the core, characterised in that:

-   -   the synthesis of nanoparticles is performed by the precipitation        of at least one salt into a solution containing the enhancement        element and/or the doping element; next,    -   the nanoparticles thereby formed are introduced into the core of        the preform by porous impregnation or modified chemical vapour        deposition (MCVD).

Using the method of the invention, it is possible to manufacturenanoparticles with various compositions, which are sturdied whenincorporated into a glass matrix, assuming that the doping elements andenhancement elements form nanoparticles with a relatively largestructure and size, between 1 and 500 nm inclusive, and preferablygreater than 20 nm, thereby making them less volatile and less sensitiveto temperature than if said elements had been inserted using othermethods.

Furthermore, the doping and/or enhancement elements are in the form ofoxides so that they are less sensitive to the high temperatures thatresult from the steps that are specific to the MCVD method, which areused afterward to manufacture a preform and transform it into the fibrecontaining said nanoparticles.

In one embodiment, the precipitation of the nanoparticles is performedunder soft chemistry conditions, particularly at ambient pressure. Inother words, the experimental means required by the process areinexpensive.

In one embodiment, the precipitation is performed within a controlled-pHsolution, such as one depending on the saturation thresholds of thevarious elements involved.

Finally, the invention also relates to an optical fibre including acore, made up of a transparent matrix and of nanoparticles that includea doping element and an element that enhances the use of said dopingagent, as well as an outer cladding surrounding the core, created byproducing the fibre from a manufactured preform using a method thatcomplies with one of the preceding embodiments.

DESCRIPTION OF FIGURES

Other characteristics and benefits of the invention will become clearupon examining the description below, which is given for illustrativepurposes and is non-limiting, of embodiments of the invention which makereference to the attached figures, in which:

FIGS. 1 a and 1 b, described above, are representative diagrams of thegain generated by fibres comprised of antimony or bismuth,

FIG. 2 depicts an amplifying fibre that complies with the invention,

FIGS. 3 a, 3 b and 3 c are structural diagrams of various glassescreated under various methods, and

FIG. 4 is a representative diagram of the gain generated by anamplifying fibre that complies with the invention.

DETAILED DESCRIPTION OF THE INVENTION

Types of glass that include bismuth or antimony have particularlyinteresting characteristics described below with the help of FIGS. 1 aand 1 b, which represent amplification gains for materials comprisingerbium as the doping element and antimony (FIG. 1 a) or bismuth (FIG. 1b) as the enhancement element.

More precisely, these FIGS. 1 a and 1 b represent the material'samplification gain (y-axis 10) as a function of the wavelength of theamplified signal (x-axis 12) for fibres comprised of antimony (curve 14,FIG. 1 a) or bismuth (curve 16, FIG. 1 b), these gains being compared tothose of a known fibre comprising aluminium as a doping agent (curve18).

It thereupon becomes apparent that the respective properties of antimony(Sb) and bismuth (Bi) are interesting for processing an optical signal,to with expanding the curve (Sb) of the gain medium's gain, orflattening said curve (Bi).

Thus, the usage of a filter intended to flatten the gain of a fibre islower when the curve of the gain for that fibre is flattened. As thisreduction may limit the pumping energy required by said fibre by about25%, it becomes clear the operating cost of the fibre is significantlyreduced.

Furthermore, simulations show that enlarging the gain band of anamplifying fibre by about 15.5% of the width of that band, which may insuch a case be wider than 38 nm, as this width is required for certaintelecommunications applications—1530 to 1568 nm

FIG. 2 depicts an amplifying optical fibre 20 that complies with theinvention. It has a core 22 comprised of nanoparticles 24 that has andoping element, such as erbium, surrounded by several enhancementselements such as bismuth and/or antimony.

In accordance with the invention, the fibre 20 was obtained through afibre-manufacturing process involving a manufactured preform with thehelp of an MCVD (modified chemical vapour deposition) method, and makesit possible to incorporate said nanoparticles 24 into the core 22 bymeans of porous absorption.

At this point, it should be noted that the nanoparticles 24 withstandbeing incorporated into glass, as their size is relatively high,generally between 20 and 500 nm inclusive. What's more, certain dopingand/or enhancement elements are present in the nanoparticles in the formof oxides, which makes them less likely to be destroyed during the stepsintended for manufacturing the preform and its transformation into anoptical fibre.

These nanoparticles 24 may be generated using a method disclosed in theinvention, i.e. by precipitating salts that include the doping and/orenhancement element(s) that are to be included in the nanoparticles.

In one example pertaining to the synthesis of nanoparticles comprisingerbium (Er) as a doping element and antimony (Sb) as an enhancementelement, precipitation makes it possible to obtain antimonynanoparticles, with the doping element erbium being incorporatedafterward.

This operating mode uses an aqueous solution of potassiumhexahydroxyantimonate (KSb(OH)6) which is added to water maintained atan acidic pH in order to obtain the precipitation of nanoparticlescontaining antimony.

The solution is then agitated at ambient temperature or at 95° C. forseveral days. Nanoparticles including antimony are finally obtainedafter centrifuging the solution, washing it, and drying it in an oven at95° C.

Afterwards, said nanoparticles may incorporate erbium via an ionicexchange achieved using a solution that includes erbium chloride ErCl3in an aqueous environment or with erbium acetylacetone Er(Acac)3including water and an organic solvent.

After centrifuging and washing, the nanoparticles are then dispersed ina controlled-pH aqueous environment and introduced into the core of apreform creating with the help of modified chemical vapour deposition,or MCVD, by impregnating a layer of porous glass. This preform is thenformed into fibres using a conventional heat treatment.

In a similar manner, nanoparticles containing bismuth (Bi) in thevicinity of erbium (Er) may also be prepared.

It should be noted that the precipitation method implemented by theinvention is not suitable for precisely sensing the environment of thedoping element (erbium in this example) with respect to enhancementelements, unlike finer nanoparticle synthesis methods, such as theorganometallic synthesis described in the above-mentioned patentapplication.

An explanation pertaining to the structure of the nanoparticlesgenerated with a method disclosed in the invention, is given with thehelp of FIGS. 3 a, 3 b and 3 c for erbium/antimony doping.

FIG. 3 a schematically depicts the type structure of a doped silicateglass SiO2 obtained using a conventional MCVD method, i.e. in which thedoping agents are inserted in no particular arrangement in the form ofdissolved chloride salts. In this structure, the doping element (Er) issurrounded by a heterogeneous and unorganised matrix of silicon, whichmay include the enhancement element (Sb). However, the majority of thisenhancement element becomes volatile when exposed to high temperaturesand/or is placed too far from the doping element to interact with it.

FIG. 3 b schematically depicts a glass 105 obtained using the MOG methoddescribed above, with said glass appearing in the form of a statisticalpresence of the enhancement element (Sb) in the vicinity of the dopingelement (erbium), owing to the ability to incorporate a large proportionrelative to the doping element.

Finally, FIG. 3 c schematically depicts nanoparticles 30 obtained usingthe method disclosed in the invention. For clarity's sake, the dopingelement (Er) and the enhancement element (Sb) have been depicted asspheres, but it should be noted that, in experimental observations,these elements appear in nanoparticles in the form of oxides.

The method for manufacturing nanoparticles does not make it possible tofully control the structure and size of these particles. Notwithstandingthe variations in the structures and sizes of the nanoparticles, theirsize may be relatively large, generally between 1 and 500 nm inclusive;the results of experiments show that the amplification gain of a fibregenerated using a method discloses in the invention is verysatisfactory, as shown below with the help of FIG. 4, which depicts theoptical signal amplification gain curve (y-axis 40) as a function of thewavelength (x-axis 42) of said signal.

It becomes clear that a fibre with nanoparticles generating using amethod disclosed in the invention (curve 46) may produce a gain over abroader range of wavelengths than the fibre manufactured using aconventional method and without a doping element.

The method of the present invention may be embodied in numerousdifferent ways. In fact, synthesising nanoparticles throughprecipitation makes it possible to generate numerous types ofnanoparticles based on various doping elements, such as erbium, andvarious enhancement elements, such as bismuth or antimony.

Additionally, a method disclosed in the invention may be implemented tomanufacture nanoparticles using the same element as both a dopingelement and an enhancement element.

Furthermore, a method disclosed in the invention makes it possible toforesee synthesising nanoparticles that include various doping and/orenhancement elements, such as: Te, Ta, Zr, V, Pb, Nb, W, In, Ga, Sn, Mo,B, As, Ti.

Additionally, a fibre that complies with the invention may include,besides the nanoparticles, elements such as aluminium, which improve thefibre's gain.

Finally, it must be emphasised that there may be many applications foran amplifying fibre that complies with the invention. As an example,such a fibre may be implemented as a Raman amplification fibre, as aRaman laser fibre, as a highly non-linear fibre, as a saturableabsorbent fibre and/or as a polarisable fibre.

1. An optical fibre including a gain medium with a core made up of atransparent material and of nanoparticles comprising a doping elementand at least one element that enhances the usage of said doping element,and of an outer cladding surrounding the core, wherein the dopingelement is erbium (Er), the enhancement element is chosen from amongbismuth (Bi) and a combination of antimony (Sb) and of bismuth (Bi), anda size of the nanoparticles is greater than 20 nanometres.
 2. An opticalfibre according to claim 1, in which the doping element and/or theenhancement element appears in the form of oxides.
 3. An optical fibreaccording to claim 1, comprising aluminium in its core, near thenanoparticles.